Four days after Ryzen CPUs officially hit the market, you can easily pick one up at Newegg or Amazon. Both sites have the new AMD processors in stock, although Newegg has temporarily run out of the top-end 1800X model. Unfortunately, you may have a hard time finding a place to put your cutting-edge octo-core CPU. Both storefronts are almost completely sold out of Socket AM4 motherboards.
If you're itching to build a machine to celebrate AMD's return to CPU relevance, your only options at those shops are a single board each from MSI, ASRock, and Asus. Your best bet might instead be to head to an actual brick-and-mortar store. Micro Center shows healthy stock on several motherboards at its locations, as long as you live nearby. The store isn't offering any of its famously-excellent CPU-and-motherboard bundles for Ryzen yet, though.
Looking around the web, many shops haven't even begun to stock boards with the AM4 socket. This could be because of limited availability, considering that the giants of e-tail can't seem to keep them in stock. It's possible that the motherboard vendors were playing it cautious, given how AMD's last CPU release turned out. It's also possible that they simply weren't prepared for Ryzen's popularity. Either way, it looks like Socket AM4 motherboards could be slim pickings for a while.